Probably defective. What is it that doesn't work? Is it the tray that refuses to open or is it that it cannot read CDs sometimes? If it refuses to read it could be the media that is dirty (scratches or fingerprints). By the way, you can also upgrade the firmware on some drives so I would check with Acer or on the web for your exact drive model to see if there's a new firmware version.
In any case, DVD burners are fairly cheap nowadays, you can get one for 20$ so if you run into too many problems just replace it.